This application claims priority from Applicant""s co-pending Canadian Patent Application No. 2,294,555, filed on Dec. 30, 1999, and entitled xe2x80x9cOptimization of a Communications System Based on Identification of an Optical Mediumxe2x80x9d.
Not Applicable.
The present invention relates to high-speed data communications systems and in particular to optimization of a data communications system based on identification of optical fiber media connected to the data communications system.
It is well known that optical signals are degraded between a transmitter and a receiver of a data communications network, due, at least in part, to signal corruption introduced by the optical fiber medium linking the nodes of the network. Commonly referred to as xe2x80x9cchannel effectxe2x80x9d, this signal corruption is normally attributable to such phenomena as attenuation and dispersion. Channel effects are influenced by such factors as manufacturing methodology, material composition and physical properties of the fiber medium, and thus commonly vary from one manufacturer to another, and even between production runs by the same manufacturer.
Attenuation is the loss of signal intensity as the light propagates through the fiber medium, and is also known as fiber-loss. Attenuation is generally an effect of the bulk properties of the fiber (nominally, its xe2x80x9ctransparencyxe2x80x9d), and may exhibit some wavelength-dependency.
Dispersion is the chromatic or wavelength dependence of a speed of travel of light through a fiber. Dispersion produces signal distortion resulting from different wavelengths of light within a pulse travelling at different speeds through the fiber medium. Signal distortion may also be caused by some parts of a light pulse following longer paths (modes) than others. Most fiber media transmit at least one wavelength (or band of wavelengths) for which little or no dispersion. Optical signals at frequencies outside this minimal-dispersion band are subject to at least some dispersion.
The total amount of attenuation and dispersion that occurs within a fiber communications link, for any particular wavelength, varies with a length of the link. Thus it is common to define the transmission characteristics of a fiber medium in terms of a total dispersion per unit length (typically per kilometer) and an average attenuation per unit length. Since these transmission characteristics are (particularly with respect to dispersion) wavelength dependent, values are normally provided for each of a range of different wavelengths.
In data communications networks with low transmission rates (e.g. less than 10 Gb/s), differences between the transmission characteristics of different fiber media do not impose serious limitations on network performance. In these systems, bit error rates are largely dependent on the communications equipment at each end of a fiber communications link. In such cases, substitution of one fiber medium having certain transmission characteristics with another fiber medium having slightly different transmission characteristics, typically will not have a major impact on the performance of the systems.
In the last decade however, transmission rates of data signals have increased dramatically. Simultaneously, the demand for ever-longer fiber spans between nodes and/or repeaters has increased. The result of these combined demands for higher data transmission rates and longer fiber spans has been a requirement for receivers with increased sensitivity. At high transmission rates, such as at 10-40 Gb/s, control of signal corruption introduced by channel effects is essential, because the transmission characteristics of a fiber have a critical bearing on the performance of the link. In order to optimize data transmission across any link, system parameters such as launched power level, peak power level, modulation shape, and wavelength plan (at both the transmitting and receiving ends of the link) need to be adjusted in accordance with the specific transmission characteristics of the fiber media through which the signal is propagated.
Manufacturers of fiber optic cables typically test the transmission characteristics of thier fiber media, either during or immediately following manufacture of the cable, and prior to delivery of the cable to a customer. Normally, this information is passed on to the customer as part of the cable delivery contract. However, operating companies often fail to maintain accurate records of the transmission characteristics of fibers that are installed in any particular cable. Companies merge, lease fiber and cables to other companies, records get lost, erroneous information gets entered into the records, and further errors occur in transferring information from the records to equipment in the field. On longer spans (up to 100 km or more) fibers having different transmission characteristics may be spliced in sequence, due to merged networks, or splicing errors. Some spans are formed using xe2x80x9cdispersion managed cablexe2x80x9d that contains fibers having differing transmission properties, deliberately spliced to each other in a particular sequence. As a result, the transmission properties of any particular fiber installed in a network are generally not known, even in cases where these transmission properties were determined by the manufacturer prior to delivery of the cable.
Normally, laboratory test instruments available for measuring dispersion are unsuitable for use with installed fiber, because they commonly require both ends of the fiber to be at the same location. Additionally, many test instruments cannot be used while a data signal is present at the same wavelength. The length of installed cables can only be very roughly determined from operating company records. More precise measurements of fiber length can be obtained from an optical time domain reflectometer temporarily attached to one end of the fiber (see IEEE Journal of Light Wave Technology, Volume 7, No. 8, August 1989, pages 1217-1224). However, optical communications systems, for example Wave Division Multiplexed (WDM) systems use photonic switching algorithms that can create dynamically varying fiber transmission paths. Such systems cannot rely upon slow and potentially inaccurate manual entry of the length and transmission characteristics of each of the fibers to which the system is connected.
Accordingly, there remains a need for a means by which an optical communications system can obtain an identification of a fiber media and/or automatically discover the transmission characteristics of optical fibers to which it is connected, and efficiently optimize one or more performance parameters in accordance with the identified transmission characteristics.
An object of the present invention is to provide an optical communications system capable of obtaining an identification of an optical fiber link, and adjusting one or more system parameters in accordance with predetermined optimum settings associated with the identification.
A further object of the present invention is to provide a an optical communications system capable automatically discovering fiber transmission properties of a fiber medium connected to the system, and adjusting one or more system parameters in accordance with predetermined optimum settings associated with the discovered fiber transmission properties.
Accordingly, an aspect of the present invention provides a method of optimizing one or more system parameters of an optical communications system adapted for connection to an optical fiber medium of an optical communications network; the method comprising the steps of: obtaining a class ID respecting the optical fiber medium; obtaining a respective optimum setting of each system parameter on a basis of the fiber identification; and adjusting a respective value of each system parameter in accordance with the respective optimum setting
A further aspect of the present invention provides an apparatus for optimizing one or more system parameters of an optical communications system adapted for connection to an optical fiber medium of an optical communications network; the apparatus comprising: means for obtaining a class ID respecting the optical fiber medium; means for obtaining a respective optimum setting of each system parameter on a basis of the class ID; and means for adjusting a respective value of each system parameter in accordance with the respective optimum setting.
A still further aspect of the present invention provides optical communications system adapted for connection to an optical fiber medium of an optical communications network; the optical communications system comprising: a transceiver including a port connected for bi-directional communications through the optical fiber medium; a controller unit for controlling operation of the optical communications system, the controller unit being adapted to adjust one or more system parameters of the transceiver in accordance with predetermined properties of the optical fiber medium. The controller unit is further adapted to: obtain a class ID respecting the optical fiber medium; obtain a respective optimum setting of each system parameter, based on the class ID; and adjust a respective value of each system parameter in accordance with the corresponding optimum setting.
In an embodiment of the invention, the step of obtaining a class ID respecting the optical fiber medium comprises a step of receiving a fiber ID respecting the optical fiber medium. The fiber ID may manually entered into the optical communications system. Alternatively, the step of receiving a fiber ID can comprises the steps of: probing the optical output of a fiber for the presence of a Bragg grating; and if a Bragg grating is detected, reading information related to the fiber ID from the optical output of the Bragg grating. Preferably, the fiber ID is used to query a cross-reference table that includes a list of fiber ID""s and a class ID associated with each fiber ID, to obtain the class ID respecting the optical fiber medium.
In embodiments of the invention, the step of obtaining a class ID comprises the steps of: discovering a value of at least one fiber transmission property of the optical fiber medium; providing a class definition table comprising a plurality of class definitions, each class definition including a respective class ID and at least one corresponding characteristic transmission property value; and selecting a class ID from the class definition table based on a closest match between corresponding ones of the at least one fiber transmission property value and the at least one characteristic transmission property value.
Each characteristic transmission property value preferably comprises a respective nominal value, and an allowable tolerance defining a value range of the characteristic transmission property. An alarm can be raised if any one fiber transmission property value does not lay within the value range of the corresponding characteristic transmission property, for any of the plurality of class definitions of the class definition table.
A value of at least one fiber transmission property can be obtained by testing the optical fiber link in situ, or alternatively prior to installation.
Preferably, the step of obtaining a value of at least one fiber transmission property comprises obtaining a respective value of any one or more of: a total optical signal dispersion; a zero dispersion wavelength; an average optical signal attenuation; a length of the fiber; a total dispersion per unit length of the fiber; and an average attenuation per unit length of the fiber.
The at least one characteristic transmission property value preferably comprises a respective value of any one or more of: the zero dispersion wavelength; the total dispersion per unit length of the fiber; and the average attenuation per unit length of the fiber. Still more preferably, the at least one characteristic transmission property value comprises a respective value of each one of the zero dispersion wavelength and the total dispersion per unit length of the fiber.
In embodiments of the invention, the step of selecting a class ID comprises the steps of: comparing each respective fiber transmission property value to a corresponding characteristic transmission property value within each class definition of the class definition table; and selecting the class ID of a one of the plurality of class definitions for which each characteristic transmission property value most closely matches a corresponding fiber transmission property value.
In embodiments of the invention, the step of obtaining a respective optimum setting of each system parameter comprises a step of searching a system table comprising a plurality of system definitions, each system definition including a respective class ID and a corresponding optimum setting for each parameter. Preferably, all of the system definitions of the system table pertain to a predetermined set of one or more related optical communications systems.
The one or more system parameters may comprise any one or more of: a transmission wavelength; a signal power; and a received signal detection threshold.
In embodiments of the invention, the step of adjusting a respective value of each system parameter comprises a step of adjusting a transmission wavelength of one or more lasers of the optical communications system. The transmission wavelength of each laser is preferably adjusted independently, and may be accomplished by tuning or by means of one or more filters.
In embodiments of the invention, the step of adjusting a respective value of each system parameter comprises a step of adjusting a signal power of one or more lasers of the optical communications system. The signal power of each laser is preferably adjusted independently.
In embodiments of the invention, the step of adjusting a respective value of each system parameter comprises a step of adjusting a detection threshold of one or more optical signal detectors of the optical communications system. The detection threshold of each optical signal detector is preferably adjusted independently.